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Faraday Cage For Lightning Protection

  1. Jun 8, 2015 #1

    anorlunda

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    A recent thread talked about a radio inside a metal box. That reminds me of a question I never resolved.

    9 years ago my sailboat was struck by lightning. 6 people were on board, but nobody was hurt, luckily. My mast is grounded to an underwater plate via a thick copper cable inside a plastic conduit. But the EMP fried all my electronics including hand held devices. Since then, I try to protect my devices by putting them in the oven of my stainless steel stove, or in cookie tins. I have no proof of these protections being effective.

    Lightning has many frequencies. My goal is not perfect shielding, but just enough attenuation of the EMP to let my devices survive. I expect that there is a lot of military research on this topic, but I don't have any references, or any way to compare weapons EMP with lightning EMP strengths.

    Ovens and cookie tins, effective or old wives tales?
     
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  3. Jun 8, 2015 #2

    berkeman

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    Do you have a microwave oven on-board?
     
  4. Jun 8, 2015 #3

    anorlunda

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    Sorry, no microwave,
     
  5. Jun 8, 2015 #4
    It is the magnetic field around the lightning bolt that is harming electronics. Its rapid rise induces voltages in current loops. Connected devices may have large loops, but also on a circuit board the induced voltages may be large enough to damage sensitive components.
     
  6. Jun 8, 2015 #5

    berkeman

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    There are a couple of things that you need to do to make an enclosure RF-tight against EM fields. I'll post some of those tomorrow (I'm at home now with a slow Internet connection).
     
  7. Jun 8, 2015 #6

    anorlunda

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    Thanks Berkeman, I'll look forward to it. But are the RF frequencies the most damaging ones?
     
  8. Jun 8, 2015 #7

    berkeman

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    I don't know much about the characteristics of EMP, so I don't know if the E or B field pulse and ringout does the most damage. But regardless, good RF shielding is the thing that is needed for keeping either out of a controlled volume. The biggest issue is sealing the joints/seams in the enclosure. For industrial RF shielding we use RF gasketing (spring finger strips) to help seal seams. I'm not sure yet what the easiest way is to replicate that cheaply in a home-brew enclosure... :smile:
     
  9. Jun 8, 2015 #8

    nsaspook

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  10. Jun 8, 2015 #9

    berkeman

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  11. Jun 8, 2015 #10

    berkeman

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    LOL -- seal sealing...
     
  12. Jun 9, 2015 #11

    anorlunda

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    Thank you napsaspook. You lived up to your handle. That is exactly the info my OP sought. (Plus bonus info on HEMP high altitude EMP)

    Although a garbage can is hard to fit in the living space of a sailboat, the cookie tins might work if properly sealed. A quick search on Amazon shows that Duck brand aluminum tape is readily available.

    I'll abandon my use of the oven. That would be nearly impossible to seal.

    A regular size cookie tin can hold a couple of phones and a tablet, or a handheld VHF, or a small GPS chartplotter. Larger, or panel mounted equipment like VHF radio, SSB radio, radar and large GPS chartplotters, are too difficult to protect, But for coastal and inshore use, apps on my phone give me backups for GPS chartplotter, weather radar, and ship traffic AIS. My conclusion is that the small devices are the easiest and most critical ones to protect from lightning.

    LED lights are highly vulnerable, and difficult to dismount to put inside a tin. I have oil lamps to back up cabin lights, and a weather proof oil lamp for use outside. To back up the red/green navigation lights, I use battery operated LED lights. I'll store those in a metal tape sealed tin. Batteries are removed from the devices and stored in the tins inside sandwich bags (in case of battery leakage).

    There is also something called a Luci light. It is an inflatable LED LIGHT/solar panel. When deflated, it stores in the size and shape of a short stack of DVD discs. I'll put one of those in a tin.

    20141007143125-LuciOriginal-3Inflating_hr_.jpg

    Plastic packing korns in the tins provide the non-conductive foam bed NASA specified, and prevent rattles. Tins of various shapes and sizes are readily available online (and they come filled with delicious stuff :-)

    Thanks everyone.
     
    Last edited by a moderator: Apr 18, 2017
  13. Jun 9, 2015 #12

    anorlunda

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    Whoops, I forgot a critical item. I also have a 12v solenoid propane shut-off valve installed near the propane tanks. That too may be fried by lightning. I have a spare solenoid that can be stored in a tin. But that may not help if the alternator, solar panels, solar panel, charge controller and even the main batteries are fried. I'll have to figure out a way to manually operate that solenoid plunger as a backup, so we can continue to cook without electricity.

    Oh in case you ask, I do have contingency plans to plug holes in the hull that might be blasted out by lightning. My interest is not academic; it is survival in an inherently self-sufficient scenario. Next to getting run over by a ship,and ground up in the ships propeller (for which I have no contingency plan), lightning is the next most likely life-threatening senario I face in blue water.
     
  14. Jun 9, 2015 #13

    berkeman

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    For sealing up seams (like between your tin and the lid), you should be using some sort of EMI gasketing material like spring fingers. To make a good conductive seal, you need 2 things:

    -1- Many points of contact

    -2- Air-tight contacts

    You don't get that generally with conductive tape, so it's best to use spring-finger gasketing. There are lots of styles, but in your case you would want to use some that has pointy contacts that face both ways. You stick the strip to the tin near the top, and then when you put on the lid, it compresses the pointy things and makes multiple contacts around the circumference of the tin. Obviously the tin and lid need to be unpainted and uncoated in the area where the contacts are being made.

    http://hollandshielding.com/content/2400/2400-Twisted-Fingerstrip-main-image.png
    2400-Twisted-Fingerstrip-main-image.png
     
  15. Jun 10, 2015 #14
    Lightning protection is tricky.

    Some rules of thumb include using a direct grounding path and a well insulated Faraday cage which is grounded through an inductor and possibly a large resistor.

    The most damaging part of lightning is the sudden surge of current. This current will often ionize air rather than follow a conductor around things. So keep your grounding path as straight as possible. Conversely, a looping path to ground with your Faraday cage will slow sudden current changes. (That assumes you need to ground your cage. Your millage may vary on this point.)

    Amazon sells Faraday Bags.
     
  16. Jun 10, 2015 #15

    berkeman

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    Good idea -- put each thing in a Faraday bag before putting it in the Faraday cage/tin. :smile:
     
  17. Jun 10, 2015 #16

    meBigGuy

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    Why isn't a small microwave, as a container, a good place to start (search microwave as faraday cage)? Seems really efficient size wise. Nice door, too. I don't know how good the shielding is, but that plus Faraday bags (what are new to me) seems like a good possibility. Also, you could give it its own ground.
     
  18. Jun 11, 2015 #17

    Baluncore

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    The good conductivity of your lead to the keel connection results in a high current with a fast risetime and so a high magnetic field.

    If you split the down lead into four identical parallel leads at the outside corners of the cabin then the fields inside the rectangle of conductors would tend to cancel. The cabin would then be a safer place to keep sensitive items.
     
  19. Jun 11, 2015 #18

    jim hardy

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    Imagine the rod below is a lightning bolt.
    Ten kiloamps is only a medium-large one and lightning's rise time is in microseconds.

    While your Faraday cage stops an E-field, i think it needs to be conductive enough so that substantial Lenz's Law currents can flow in it opposing the B-field that encircles the lightning bolt.
    That'll reduce magnetic induction inside the cage.
    So while your oven may not be a perfect Faraday enclosure i think it'll help provided its metal case is thick enough to allow a lot of current flow.
    I'm not sure whether a ferromagnetic skin would help further, but the galvanized iron garbage can suggested above seems intuitively "right"..

    Biot.PNG


    Thirty years ago i had to figure out why a lightning strike tripped our power plant. Induction , Biot Savart.
     
    Last edited: Jun 11, 2015
  20. Jun 11, 2015 #19
    I would advise against this. While it would work in theory, I suspect in practice most of the current would flow down one conductor. The return stroke would then flow up another creating a loop and amplifying the problem.

    With some experimentation I think the system could be designed to work, but that sounds more like a research grant than practical lightning protection.
     
  21. Jun 11, 2015 #20
    I like the microwave idea. The size is convenient. Plus you get a microwave to cook diner. I suspect a Faraday bag in a microwave is plenty of protection. Microwave ovens are designed as Faraday cages. The frequencies of lightning are lower (~100kHz, ultra wide band), But I don't think that's a problem.

    Still, if you have the extra room for the trash can on a small boat, use that. Trash cans are made of iron which isn't as good a conductor, OTOH the magnetic properties should channel the field around the interior. Both are good ideas.
     
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